The present invention relates to amplification of AC fluidic signals and more particularly to a method for reduction of flow noise and DC bias between stages of Laminar Proportional Amplifiers.
The technology known as fluidics provides sensing, computing, and controlling functions with fluid power through the interaction of fluid (liquid or gas) streams. Consequently, fluidics can perform these functions without mechanical moving parts. The inherent advantages of fluidics are, therefore, simplicity and reliability, since there are no moving parts.
Since 1970, a number of important applications of fluidics have been realized. The areas of use include the aerospace industry, medicine, personal-use items, and factory automation. Fluidics for military systems has also progressed to the point where several systems are now in use. In most cases, the reason for selecting fluidics has been a combination of low cost, high reliability, inherent safety, and the ability to operate in severe environments.
Almost all early (first generation) fluidic devices were operated in the turbulent-flow regime. Since the mid-1970's, the emphasis has shifted to the use of laminar-flow (second generation) fluidic components. Turbulent flow is characterized by a "noisy" jet; in contrast, laminar flow is characterized by a "quiet" well-defined jet. Laminar-flow fluidic devices are used primarily in signal applications where the ability to detect and process extremely small pressure signals is essential.
Most fluidic amplifiers have at least four basic functional parts. These include (1) a supply port, (2) one or more control ports, (3) one or more output ports, and (4) an interaction region. These sections may be compared, respectively, to the cathode, control grid, plate, and interelectrode region of a vacuum tube. Many fluidic amplifiers also contain vents to isolate the effects of output loading from the control flow characteristics.
The supply jet in the fluidic amplifier passes into the interaction region where it is directed toward the output port(s) or receiver(s). Control flow injected into the interaction region determines the direction and distribution of the supply flow, which in turn affects the flow reaching the receiver(s). The amount of pressure or flow recovery available in a receiver is determined by the internal shape of the device. Useful amplification occurs inasmuch as change in output energies can be achieved with small changes in control energies.
Laminar proportional amplifiers (LPA's) and sensors are active (flow consuming) devices that form the building blocks of fluidic control systems. A typical application requires several of these active devices interconnected to perform a specific control function. Generally they are packaged to provide a means to interconnect these devices, distribute the supply and vent flow, and accommodate additional components such as flow restrictors and volumes required to accomplish various control functions. The most convenient configuration is to use a planar element format which has two flat sides.
Staging is the process of connecting two or more amplifiers in series to obtain an increase in gain. An LPA has a pressure gain, i.e., a small change in pressure at the inputs produces a larger change in pressure at the outputs. The pressure gain is at a maximum when no flow is delivered at the outputs (blocked load). Pressure gain decreases as flow is withdrawn from the amplifier outputs. If the amplifier outputs are wide open, the pressure gain is essentially zero. An LPA also has flow gain; a small change in flow at the inputs produces a larger change in flow at the outputs. Flow gain is maximum when the amplifier outputs are wide open, and is zero when the amplifier is operated block loaded. Since power is defined as the product of pressure and flow, an LPA also has power gain.
Of the three gains described above, staging for pressure gain is the most common requirement. There are several methods of staging LPA's to obtain pressure gain. For example, amplifiers can be self-staged by connecting identical elements all operating at the same supply pressure. This practice is convenient for assembly and manifolding and for maximizing the input/output resistance ratio; however, dynamic range is not optimized. Dynamic range is related to the maximum available output signal which, for LPA's, increases with an increase in supply pressure. If two identical amplifiers operating at the same supply pressure are staged, the first amplifier will saturate the second amplifier before the first amplifier reaches its own saturation level. Thus, the full dynamic range of the first amplifier is not being used. In some applications, the single-stage amplifier dynamic range is high enough so that a self-staged reduction in dynamic range can be tolerated.
Thus it can be seen from the above discussion that it is well known in the art that LPA's can be staged to form gainblocks with high pressure gain and dynamic range. In a conventionally staged gainblock, the output flows from the previous stage are directed to flow completely into the input ports of the next stage. As a result, flow noise is generated within the interconnection region by the interaction of the fluid molecules with the wall of the interconnection passage and this flow noise is amplified by the next stage. This amplified flow noise can significantly reduce the signal-to-noise ratio in the output signal. In addition to flow noise, DC bias, present in any conventionally staged gainblock, interferes with the output signal. When LPA's are staged to form a gainblock, the DC bias from the previous stage can saturate the gainblock. It is, therefore, very difficult to build a gainblock with more than four stages without carefully matching the bias characteristics of each LPA.
When conventionally staged gainblocks are used to sense very low pressure signals, the interstage flow noise generally overwhelms the input signals. As a result, it is impossible to detect any output signal at the output ports without signal processing and filtering. It can be seen, therefore, that there is a great need to devise a new staging method for LPA's so that interstage flow noise and DC bias is reduced or eliminated.